I. Field of the Invention
The present invention relates to data communication. More particularly, the present invention relates to novel and improved techniques for controlling transmit power of multiple channels in a CDMA communication system (e.g., a W-CDMA system).
II. Description of the Related Art
In a wireless communication system, a user with a user terminal (e.g., a cellular phone) communicates with another user through transmissions on the downlink and uplink via one or more base stations. The downlink (i.e., forward link) refers to transmission from the base station to the user terminal, and the uplink (i.e., reverse link) refers to transmission from the user terminal to the base station. The downlink and uplink are typically allocated different frequencies.
In a Code Division Multiple Access (CDMA) system, the total transmit power from a base station is typically indicative of the total downlink capacity since data may be concurrently transmitted to a number of users over the same frequency band. A portion of the total transmit power is allocated to each active user such that the aggregate transmit power for all users is less than or equal to the total available transmit power.
To maximize the downlink capacity, the transmit power to each user terminal may be controlled by a power control loop such that the signal quality, as measured by the signal-to-noise-plus-interference ratio (SNR), of a transmission received at the user terminal is maintained at a target SNR. This target SNR is often referred to as the power control setpoint (or simply, the setpoint). A second power control loop is typically employed to adjust the setpoint such that a desired level of performance, as measured by a frame error rate (FER), is maintained. The downlink power control mechanism thus attempts to reduce power consumption and interference while maintaining the desired link performance. This results in increased system capacity and reduced delays in serving users.
Many newer generation CDMA systems support concurrently transmission on multiple channels to provide high-speed data service and/or multiple services (e.g., voice and packet data). These channels may be used to transmit data at different data rates, and may further utilize different processing schemes. A feedback stream (or power control subchannel) may be allocated to each user terminal for power control of these channels. The feedback stream is typically used to send information indicative of the received signal quality for the transmission on one of the channels. This information may then be used by the base station to provide power control for all channels.
The power control becomes more challenging if the transmit power for multiple channels is not related by a defined relationship. This can result if the channels are not transmitted from the same set of base stations (i.e., different “handover” situation). For example, a first channel may be transmitted from a set of base stations using soft handoff, and a second channel may be transmitted from only one base station in the set. For the first channel, the user terminal collects and combines the transmit power from all transmitting base stations to recover the transmission, and power control for this channel is based on the combined power. And for the second channel, the power control should be based on the transmit power received from the single transmitting base station.
From the perspective of the base station transmitting the second channel, the transmit power for the two channels may not be correlated. Typically, the percentage contribution from individual base stations is not known for a channel in soft handoff. Thus, the amount this base station contributes to the first channel may not be known. If a single feedback stream is allocated and used to send power control information for the first channel, effective power control of the second channel is typically not possible based on this feedback stream. If the transmit power for the two channels is not correlated, the base station is unable to accurately adjust the transmit power for the second channel based on feedback information for the first channel.
As can be seen, techniques that can be used to effectively control the transmit power of multiple channels, which may be transmitted from different sets of base stations, are highly desirable.